In the destruction of pollutants through combustion in a gas treatment installation of the type known as a combustion exchanger (see US-A-4 741 690), electric heaters are often used to maintain the correct operational temperature in the middle of the combustion bed. If the chemical energy developed in the form of heat during the destruction of the pollutants is sufficient, there is, however, no need for supply of electric energy. The installation is, in this case, self-sustaining and the electric heaters need only be used to start the operation of the installation. The start-up requirement is that the temperature to sufficiently high to ensure continuation of the reaction of destruction. Once the reaction- has started, it generates by itself the heat required to maintain the temperature level, provided the chemical energy from the reacting pollutants is sufficient.

When the energy emanating from the pollutant is insufficient to maintain the operational temperature in the bed at the required level, it is possible, instead of using the electric heaters, to mix additional fuel into the air to be cleaned, in order to provide the required heat. For instance, natural gas or propane could be added to the polluted stream of air. Also vaporized solvents are sometimes used. Often the prices of energy make it less expensive to add fuel than to resort to heating by electricity.

The dimensions of the electric heating system in an electrically heated combustion exchanger generally are chosen to ensure that the installation can be operated at full air flow and maintain the operational temperature, also when the air temporarily contains no pollutants that generate energy. In other words, the installation should be able to function also with clean air. One consequence of this is that the electric connections must be dimensioned for considerable power requirements. However,

it is costly to have to provide for considerable power for the electric heaters with consequential large connected power, which means considerable costs, although the equipment perhaps is in use only very unfrequently. The use of equipment designed to supply extra fuel to the strem of air to be cleaned may, under these circumstances, result in cost savings, since it becomes possible to reduce the electric power connections. This is prior-art technology and is used in many cases in conjunction with conventional combustion exchangers.

In a conventionally constructed, electrically heated combustion exchanger, the electric heaters are positioned in the hottest zone of the bed. In operation, the tem¬ perature is high in this zone (about 1000°C), with con- sequential high demands on the corrosive resistance of the materials of which the heaters are made. This is true whether the heat is supplied by electricity, by the pollutants or by the fuel added to the flow. The problem is identical when using other types of heaters than electric heaters.

By means of the invention defined in the following a combustion exchanger may be arranged in such a manner that any heaters incorporated therein are spared long-term presence in high temperatures. One embodiment of the invention is illustrated in Fig. 1.

The incoming air to be cleaned enters through a conduit 1 and is supplied to an air-direction changing apparatus 2 in which the air is alternatingly directed upwards to an air distribution space 3 and downwards to an air distribution space 4. From there, the air passes through a combustion bed 5 in which the chemical reaction takes place. Via the air distribution space 4, alter¬ natingly the air distribution space 3, the air then returns to the air-direction changing apparatus 2 and finally the cleaned air exits thorugh a conduit 6. For the purpose of maintaining a hot zone in the middle of the bed 5 the direction of the air is changed at regular

intervals, in accordance with the principle described in the US Patent Specification referred to in the afore¬ going. During normal operating conditions, heat losses are compensated for by the heat developed in the reaction process with the pollutants and/or with the fuels mixed with the air for this very purpose.

A higher temperature is, however, required initially in order to initiate the chemical reactions. The necessary starting temperature in the bed is achieved by means of one or several heaters 7 which are positioned vertically out of center, i.e. displaced from the central part of the bed, and which preferably are located at the upper part of the bed.

When the installation is to be started-up from cold conditions, the heaters 7 are activated. They are used to heat a layer in the bed up to a starting temperature which need not be as high as the normal operational temperature. It is sufficient if the temperature level is such as to ensure that the mixture of pollutants/fuel and air starts to react during its passage through the layer and that the reaction progresses to a point where more heat is devel¬ oped inside the layer than is removed therefrom. The temperature inside the layer will therefore rise while the installation is in operation. When the correct start-up temperature level has been achieved by means of the heaters, the latter thus are stopped and by starting the supply of the air flow and the supply of pollutants/fuel, the temperature of the hottest layer will soon increase up to the normal operational temperature. Because the path from the hot layer to the environ¬ ment is shorter towards the upper side of the bed than towards the lower one, the layer will lose more heat upwards than downwards. Consequently, the heat has a tendency to move downwards and eventually it will be centered in the bed.

By controlling the air-directing mechanism in such a manner that a larger volume of air passes downwards through the bed than upwards, this process may be accel¬ erated and become so rapid that the heaters, having starionary positions inside the bed, will never be exposed to the normal working temperature. The simplest way to achieve this is to allow the flow to pass upwards and downwards at intervals of mutually different lengths, but it is likewise possible to keep the interval times of equal length but instead vary the flow volumes per time unit.

With our without such predetermined control the end result will be a centration of the hottest layer to the middle of the bed and the asymmetrically located heaters will constantly find themselves in a cooler part of the bed that is positioned externally of the hottest layer. From then on, the combustion exchanger will be run, when desired with extra fuel added to the flow, without the heaters ever being exposed to the high temperature of the hottest zone.

One consequence of this is that the heaters may be made from cheaper materials and that their serviceable life increases while at the same time they will need less servicing and maintenance. In accordance with the previous exemplary embodiment of the invention the heaters are positioned asymmetrically inside the bed in such a manner that they are closer to the upper face of the bed than to the lower one. This position facilitates servicing work on or replacement of the heaters, as it eliminates the need of removing large quantities of material to make the heaters accessible. In principle, and within the scope of the invention, they could of course also or instead have been positioned in the lower part of the bed. Further, a vertical flow of air through the bed has been assumed above. Also in the case of a horizontal air flow the principle of the out-of-the- center position with respect of the heaters could, of

course, be adopted. In the latter case it should be understood that the bed arrangement is turned accordingly, whereby "upwards and downwards" are replaced by "left and right".

The most common type of heaters in combustion exchangers is electric heaters. The principle of asymmetrically positioned heaters as defined in the subject invention is applicable also to other types of heaters, such as oil or gas fuelled heaters.